This invention relates to battery disconnect switches and, in particular, to battery disconnect switches that are entirely bidirectional, i.e., capable of blocking or conducting a current in either direction.
A battery disconnect switch is a bidirectional switch that is used for enabling or disabling a current flow between a battery and a load or a battery and a battery charger. Since the battery charger may provide a voltage that is greater than the battery voltage, the switch must be able to prevent a current flow from the charger to the battery when the switch is in an off condition. Conversely, the switch must be able to block a current flow in the opposite direction when the load has a low resistance, when the load is shorted, or when a battery charger has an incorrect output voltage or is connected with the polarity of its terminals reversed.
Moreover, when the switch is turned on, in most situations the switch must be capable of conducting a current in either direction. When the battery is charging, a current flows from the battery charger through the switch to the battery. During normal operation, a current flows through the switch in the opposite direction from the battery to the load. Accordingly, the purpose of the battery disconnect switch is to provide a means to enable or interrupt current conduction between a battery and other electrical devices, regardless of the current or voltage to which the switch is exposed. Current interruption is particularly valuable to provide the following functions: prevent overcharging of a battery; prevent over-discharging of a battery; prevent excessive currents from flowing in the case of a shorted load or battery; protect a battery from improper battery charger connections (such as a reverse connection); extend battery shelf life by minimizing leakage, which will tend to discharge the battery over extended durations; and facilitate sequencing or switching between multiple batteries and multiple loads.
While conventional power MOSFETs (in which the source and body are shorted) may be used to form a battery disconnect switch, the presence of a single P-N junction diode between the drain and the source makes a single power MOSFET incapable of bidirectional current blocking. Since bidirectional current blocking between two or more power sources is a necessary function of all battery disconnect switches, the use of discrete power MOSFETs requires that two devices be placed back-to-back in series, with either a common source or a common drain. The total on-resistance of the switch is then twice that of an individual power MOSFET. Such an arrangement is shown in FIG. 1A, wherein MOSFETs 10 and 11 are connected in a common source configuration.
An alternative is to use a symmetrical drifted lateral MOSFET not having a source/body short. Such an arrangement is described in the above-referenced application Ser. No. 08/219,586, application Ser. No. 08/160,539, and application Ser. No. 08/160,560. An example of this type of arrangement is shown in FIG. 1B wherein a symmetrical MOSFET 12 has one terminal connected to the high side of battery 13 and the other terminal connected to the load. (While MOSFET 12 is symmetrical, the terminal connected to battery 13 is arbitrarily referred to as the drain, and the terminal connected to the load is arbitrarily referred to as the source.) The body of MOSFET 12 is connected to the negative terminal of battery 13, which is normally grounded, so that the drain-to-body diode within MOSFET 12 is reverse biased. The load is connected with its negative terminal directly connected to ground while its positive terminal is connected to the source of MOSFET 12. As a result, whether MOSFET 12 is off or on, its source-to-body diode, like its drain-to-body diode, remains reverse biased under all normal conditions.
MOSFET 12 offers a low on-resistance while being able to block current in both directions when it is turned off. Also, since MOSFET 12 is connected to the high side of battery 13, the grounded low side of the battery can form a common conductive plane in a printed circuit board. In many situations, this helps reduce noise and makes the wiring of the printed circuit board relatively straightforward. MOSFET 12 may be turned off easily by grounding its gate. (However, extra circuitry is required to protect against a reversed battery charger condition. An example of such circuitry is described in the above-referenced application Ser. No. 08/219,586. During conduction, the battery voltage increases the reverse bias on the source-to-body junction of MOSFET 12, leading to the well known xe2x80x9cbody effectxe2x80x9d wherein the threshold voltage of the MOSFET increases. Assuming a fixed gate drive voltage, the body effect increases the on-resistance of the device. Also, the gate of MOSFET 12 must be driven above the battery voltage to guarantee a low on-resistance. This requires a charge pump to generate a positive supply above the voltage of the battery (requiring an oscillator which consumes power).
In accordance with this invention, a bidirectional battery disconnect switch (BDS) is connected to the low side of the battery, which is typically grounded. The BDS includes a switch MOSFET which is symmetrical, having no source/body short.
Circuitry is provided to connect the body of the MOSFET to whichever of the switch MOSFET""s terminals is biased more negatively. In a preferred embodiment, this circuitry includes a pair of MOSFETs. A first MOSFET is connected between the battery-side terminal of the switch MOSFET (arbitrarily designated the drain) and the body of the switch MOSFET. A second MOSFET is connected between the body of the switch MOSFET and the load-side terminal of the switch MOSFET (arbitrarily designated the source). The gate of the first MOSFET is connected to the source of the switch MOSFET; the gate of the second MOSFET is connected to the drain of the switch MOSFET. Accordingly, when the drain of the switch MOSFET is at a higher voltage than the source of the switch MOSFET, the second MOSFET is turned on and shorts the body and source of the switch MOSFET. Conversely, when the source of the switch MOSFET is at a higher voltage than the drain of the switch MOSFET, the first MOSFET is turned on and shorts the body and drain of the switch MOSFET.
The switch MOSFET is turned on by connecting its gate to the positive battery voltage and turned off by grounding its gate. In a preferred embodiment, the gate of the switch MOSFET is tied to its body, which is grounded.
When the switch MOSFET is turned on, the voltage difference between its source and drain is relatively small, and neither of the first and second MOSFETs is turned on. This allows the body of the switch MOSFET to float. If enough current is forced into the body of the switch MOSFET while it is turned on, the body potential of the switch MOSFET could conceivably rise above the potential of both its source and drain terminals, thereby forward-biasing both its drain-to-body and source-to-body diodes. This could create an excessive source-to-drain current in the switch MOSFET and could damage the device.
A second pair of MOSFETs is used to prevent this from happening. This pair includes a third MOSFET, which is connected in parallel with the first MOSFET, and a fourth MOSFET, which is connected in parallel with the second MOSFET, the respective gates of the third and fourth MOSFETs being tied to the body of the switch MOSFET.